Tires require good wet skid resistance, low rolling resistance, tear strength, and good wear characteristics. It has traditionally been difficult to improve the wear characteristics of a tire without sacrificing wet skid resistance and traction characteristics. These properties depend, to a great extent, on the dynamic viscoelastic properties of the rubbery polymers utilized in making the tire.
In order to reduce the rolling resistance and to improve the treadwear characteristics of tires, rubbery polymers having a high rebound have traditionally been utilized in making tire tread rubber compounds. On the other hand, in order to increase the wet skid resistance of a tire, rubbery polymers that undergo a large energy loss have generally been utilized in the tread of the tire. In order to balance these two viscoelastically inconsistent properties, mixtures of various types of synthetic and natural rubber are normally utilized in tire treads. For instance, various mixtures of styrene-butadiene rubber (SBR) and polybutadiene rubber are commonly used as a rubbery material for automobile tire treads.
It is desirable for synthetic rubbers to exhibit low levels of hysteresis. This is particularly important in the case of rubbery polymers that are used in tire tread compounds. Such polymers are normally compounded with sulfur, carbon black, accelerators, antidegradants and other desired rubber chemicals and are then subsequently vulcanized or cured into the form of a useful article. The physical properties of such cured rubbery polymers depend in part upon the degree to which the fillers, such as carbon black or silica, are homogeneously dispersed throughout the polydiene rubber. This is in turn related to the affinity of the carbon black or silica to the rubber.
This homogenous dispersion, which is affected by the level of affinity between the filler and the polymer, can be of practical importance in improving the physical characteristics of rubber articles that are made utilizing polydiene rubbers. For example, the rolling resistance and tread wear characteristics of tires can be improved by increasing the affinity of carbon black to the rubbery polymers utilized therein. Therefore, it would be highly desirable to improve the affinity of a given rubbery polymer for carbon black and/or silica. This is because a better dispersion of carbon black throughout polydiene rubbers, which are utilized in compounding tire tread compositions, results in a lower hysteresis value and consequently tires made therefrom have lower rolling resistance. Accordingly, improving the affinity of the rubbery polymer to the filler, such as carbon black and silica is extremely important in reducing hysteresis.
One manner to attain improved interaction of rubbery polymers with fillers is to functionalize them. Such rubbery polymers can be functionalized with various compounds, such as amines. U.S. Pat. No. 4,935,471 discloses a process for preparing a polydiene having a high level of affinity for carbon black which comprises a capping agent selected from a group consisting of (a) halogenated nitrites, (b) heterocyclic aromatic nitrogen containing compounds, and (c) alkyl benzoates. Moreover, U.S. Pat. No. 4,935,471 discloses that lithium amides are preferred initiators. This combination provides polymer chains with polar groups at both terminal ends of the polymer chains.
Another approach to improve the affinity of a given rubbery polymer for carbon black and/or silica would be to incorporate functional groups throughout the polymeric chain. This incorporation can be achieved by forming rubbery polymers from a mixture of monomers, of which at least one of the monomers is functionalized, for example, an amine.
Therefore, what is needed are new functionalized monomers and methods for preparing and polymerizing such functionalized monomers to produce functionalized rubbery polymers, without the requirements of masking and unmasking the functional group, or of adding the functionality only after polymerization, for example.